In recent years, in accordance with requirements for the miniaturization, weight saving, and improvement of calmness of automobiles, efforts have been made for miniaturization, weight saving, and tight-sealability in engine rooms of electrical components and auxiliary machine parts thereof. On the other hand, the requirements of high output and high efficiency have been increased for the performances of the devices, and in electrical components and an auxiliary machine in an engine room, a means for making up for the decrease in the output that arises in accordance with miniaturization by rotating at a high-speed is adopted. As examples of the rolling bearing for electrical components/auxiliary machines for automobiles, general description will be given below of a rolling bearing for fan coupling devices, a rolling bearing for automobile alternators, and a rolling bearing for idler pulleys.
A fan coupling device for automobiles is a device provided with a housing including a viscous fluid enclosed therein and a blast fan attached to the outer periphery surface and with a rotor that is directly linked to an engine. In this device, the housing is connected to the engine via a bearing, which performs optimal blasting corresponding to the temperature of the engine by controlling the driving torque transmission amount from the engine and the rotation number of the fan by utilizing the shear resistance of the viscous fluid which increases or decreases in response to the atmospheric temperature. Therefore, a rolling bearing for fan coupling devices requires durability by which the rolling bearing can withstand extremely severe environments such as unevenness of rotation such that the rotation number varies from 1,000 rpm to 10,000 rpm in accordance with the variation of the engine temperature, and high-speed rotation at 10,000 rpm or more under a high temperature of 180° C. or more during high-speed operation in summer.
An alternator for automobiles has functions to generate electrical power by receiving the rotation of an engine with a belt to thereby supply an electrical power to an electrical load of a vehicle, and to charge a battery. Furthermore, an idler pulley for automobiles is used as a belt tensioner for a driving belt that transmits the rotation of an engine to an auxiliary machine of an automobile, and has a function as a pulley for providing tension force as a tensioner to a belt in the case when the distance between axes is fixed, and a function as an idler that is used for changing the running direction of the belt or for avoiding obstacles to thereby decrease the inner volume of an engine room. The alternator for automobiles and the idler pulley for automobiles also require durability by which the alternator or idler pulley can withstand an extremely severe environment of high-speed rotation at 10,000 rpm or more under a high temperature of 180° C. or more.
For the lubrication of the rolling bearings thereof, a grease is mainly used. However, when the conditions for use become severe, such as rapid acceleration and deceleration, high temperature and high-speed rotation, specific peeling associated with white tissue change may occur on a rolling surface of a rolling bearing at an early stage. This specific peeling is considered to be hydrogen brittleness, which is a breaking phenomenon generated from a relatively shallow part on the surface of a rolling surface, and is caused by hydrogen generated by the decomposition of the grease, and the like, unlike peeling from the inside of a rolling surface generally caused by metal fatigue. For example, it is considered that peeling at an early stage due to hydrogen brittleness is caused by the fact that the grease is decomposed to generate hydrogen, and the hydrogen enters the steel of a rolling bearing.
Since hydrogen significantly decreases the fatigue strength of steel, even under a condition in which contacting elements are lubricated by elastic fluid lubrication in which the elements are separated by an oil film, cracks generate around the interior of a rolling surface layer at which an alternating shear stress becomes the largest, and the cracks transmit to lead peeling at an early stage. Furthermore, it is considered that, when used under a condition in which water is mixed with the grease, under a condition associated with slipping, under a condition in which energization occurs, or the like, the water or grease is decomposed to lead easy generation of hydrogen, and the hydrogen enters the steel, whereby the above-mentioned peeling at an early stage due to hydrogen brittleness easily occurs.
In the above-mentioned bearings that are used in electrical components and auxiliary machines in automobiles, and also in bearings that support main axes of machine tools, motors in industrial machines, motors for driving electric automobiles and hybrid automobiles, and the like, a grease is mainly used for the lubrication thereof, but the condition of use thereof have been stricter year by year, since miniaturization, high performances and high outputs have been required. Accordingly, similar specific peeling is a problem also in rolling bearings used in these. Furthermore, this specific peeling has become obvious also in rolling bearings for use in oil lubrication such as rolling bearings for use in reducers for construction machines and speed-up gears for wind-power generation devices.
As methods for suppressing such specific peeling associated with white tissue change which generates at an early stage, for example, a method in which a passivator is added to a grease (see Patent Document 1) and a method in which bismuth dithiocarbamate is added (see Patent Document 2) are proposed. Furthermore, since a bearing rolling surface is constituted by a bearing steel of an iron-based metal, a method in which a metal powder of aluminum, silicon, titanium, tungsten, molybdenum, chromium, cobalt or the like is incorporated in a grease composition with consideration for the mutual solubility with iron is also suggested (see Patent Document 3).
Furthermore, in grease compositions that have been conventionally used for rolling bearings, a grease composition containing alkanolamine derivative modified with a hydrophilic group as a hydrophilic organic inhibitor is known (see Patent Document 4). This alkanolamine derivative is a salt of a dibasic acid such as dodecanoic acid or sebacic acid, an acid such as boric acid, and alkanolamine such as diethanolamine, aminotetrazole or diethylaminoethanol.
In addition, as a lubricant composition that is excellent in heat-resistance, mechanical stability, water resistance, rust resistance, loading resistance, flame retardancy and the like, a lubricant composition containing a base oil formed of mineral oils and synthetic oils, tricalcium phosphate, and a grease structure stabilizer such as diethanolamines is proposed (see Patent Document 5).